What does the sigma symbol mean in math?

No.14354824 ViewReplyOriginalReport
I am taking general chemistry/organic chemistry and I frankly don't remember anything about math beyond simple arithmetic like addition, subtraction, dividing and multiplying.
First of all, what does the equation in pic rel mean? Can someone give me an example I can write down in my notebook so I can understand it?
Second, how can I get a bootcamp/crash course on essential math for college (chemistry/pharmacy) without studying shit I don't need?